I know, but it doesn't exist in MV. Try calling it and see for yourself.

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This function appeared in rpg maker with version 1.5 (when support for es6 appeared). Also, developers added their similar function, which is called contains (see rpg_core.js). It was from version 1.0.
It is also important to know that the functions includes and indexOf work differently. It seems that indexOf does not work with NaN.

That's quite weird, I kept getting errors when I tried using it. I think I started with 1.5.1 or 1.5.2 so it should have been there (last year in November or so). Oh well. ¯\_(ツ)_/¯ I'll switch out my own code.